1. Field of the Invention
This invention relates to regulation of cell growth, and more particularly to regulation of cancer cell growth. In particular, peptides and polypeptides derived from particular regions of the syndecan 1 have been shown to inhibit engagement of α6β4 integrin by HER2, thereby limiting tissue invasion and tumor cell survival.
2. Related Art
EGFR family receptor tyrosine kinases are causal factors in cancer, especially in breast cancer. HER2 is overexpressed, typically via gene amplification, in 25-30% of breast tumors (HER2+ breast cancer), leading to poor overall survival (Berger et al., 1988; Slamon et al., 1987). Expression of the α6β4 integrin is also upregulated in cancer, often along with the expression of receptor tyrosine kinases such as HER2 that link with the integrin and activate it to drive tumor cell proliferation, survival and invasion. Identified as the TSP-180 antigen in mouse tumors (Falcioni et al., 1986), or the A9 antigen in humans (Van Waes et al., 1991; Kimmel and Carey, 1986), high expression of this antigen predicts a higher rate of early relapse in head and neck squamous cell carcinoma (Wolf et al., 1990; Carey et al., 1987). In more recent studies exploring its tumor-promoting role using animal models, keratinocytes that lack expression of the β4 integrin subunit fail to form invasive squamous cell carcinomas when transformed with ras and IκB, unlike their normal counterparts that express the integrin (Dajee et al., 2003; Tran et al., 2008). Despite this seeming importance of the integrin in squamous cell carcinoma, there currently are no therapeutics available to target its tumor-promoting activities. The α6β4 integrin is also expressed on vascular endothelial cells in vivo, where its function in hemidesmosomes allows the endothelium to resist frictional forces as it does on stratified epithelia. Giancotti has shown a clear role for α6β4 integrin in tumor angiogenesis and that α6β4 is expressed in the vasculature of several tumor types (prostate, breast, glioma, papillary thyroid, melanoma) (Nikolopoulos et al., 2004). Although not studied extensively, it clear that endothelial cells express EGFR family members, including the HER2 kinase (Amin et al., 2006).
It is increasingly appreciated that growth factor receptors and extracellular matrix receptors work closely together to regulate cell proliferation, invasion and survival, and may do so as macromolecular assemblies at the cell surface. Indeed, HER2 is known to be coupled with the α6β4 integrin and signaling from this receptor assembly is implicated in both tumorigenesis and tumor-induced angiogenesis. However, the means by which these receptors are coupled remains largely unknown. Work from a variety of laboratories has shown a linkage between the α6β4 integrin and HER2 in breast and other cancers (Folgiero et al., 2008; Lu et al., 2008). This integrin in normal cells assembles with laminin in the basement membrane underlying basal epithelial cells as well as endothelial cells lining blood vessels, forming stable hemidesmosomes in which the long (ca. 1000 amino acid) cytoplasmic domain of the β4 subunit anchors to the keratin filament network in the cytoplasm of the cell (Hopkinson and Jones, 2000; Nievers et al., 1999; Wilhelmsen et al., 2006). In contrast to this “stabilizing” role, however, the integrin takes part in the invasion, proliferation and survival of tumors that overexpress the receptor tyrosine kinases HER2, EGFR, or c-Met—leading to the assembly of these kinases with the integrin (Wilhelmsen et al., 2006; Agazie and Hayman, 2003; Mainiero et al., 1996; Mariotti et al., 2001; Bertotti et al., 2005; Bertotti et al., 2006; Bon et al., 2007; Falcioni et al., 1997; Gambaletta et al., 2000; Santoro et al., 2003; Trusolino et al., 2001; Tsuruta et al., 2008; Giancotti, 2007). When coupled with the integrin, signaling from these kinases disrupts the hemidesmosome (Rabinovitz et al., 2004; Wilhelmsen et al., 2007) and leads to tyrosine phosphorylation of the β4 cytoplasmic domain, providing docking sites for signaling effectors that drive tumor cell proliferation, invasion and survival (Wilhelmsen et al., 2006; Mariotti et al., 2001; Bertotti et al., 2006; Wilhelmsen et al., 2007; Mainiero et al., 1997; Shaw et al., 1997; Guo et al., 2006; Merdek et al., 2007; Dutta and Shaw, 2008; Datta et al., 1999; Dans et al., 2001; Shaw et al., 2001; Yang et al., 2010). The distal third of the β4 tail containing these phosphorylation sites has thus been termed the β4 “signaling domain” (Guo et al., 2006) (FIG. 1). In studies using the MMTV-Neu mouse model of HER2+ breast cancer, replacement of native β4 with a β4 mutant (β41355T) lacking this signaling domain acts as a suppressor of breast cancer (Guo et al., 2006), suggesting that the wild type β4 receptor normally couples with HER2 to drive tumorigenesis in human HER2+ breast cancer as well. Work utilizing a number of mammary carcinoma cell lines, focusing mostly on HER2+ cells, also shows that HER2/α6β4 signaling is critical for invasion and survival of these tumors (Falcioni et al., 1997; Gambaletta et al., 2000; Guo et al., 2006). Complementing their expression in the tumors, HER2 and EGFR are also expressed in endothelial cells, especially those induced by tumors (Amin et al., 2006; Bruns et al., 2000; Kedar et al., 2002), and couple with the α6β4 integrin during tumor-induced angiogenesis (Nikolopoulos et al., 2004). What remains unanswered is how the receptor tyrosine kinase associates with the integrin to initiate this process. The answer to this question could reveal significant therapeutic opportunities in the treatment of cancer.